Methods and apparatus to change the position of a film frame in a motion picture projector film gate are many and varied. Most utilize sophisticated mechanical gearing and linkage arrangements which are difficult to maintain, require frequent cleaning and adjustment and add to the noise of the projector. There has long been a need for a simple, reliable apparatus to adjust film framing in a film gate.
The apparatus disclosed herein provides for simple, reliable and quiet film frame adjustment. A mechanism for intermittently pulling down film through a motion picture projector film gate, such as a Geneva mechanism, is mounted on a moveable plate so that the pull-down mechanism can be moved relative to the film gate. By such means, the position of engagement and release of the film sprocket holes changes with respect to the film gate, so that the centering of each film frame can be appropriately adjusted.
In a particular embodiment, a somewhat elongated plate is utilized with one end pivotally mounted to the motion picture projector body and the film engaging portions of the pull-down mechanism mounted near the other end. A rack is also attached at the other end which, in conjunction with a pinion gear, is used to pivot the plate about its pivot point, thereby changing the position of the pull-down mechanism relative to the film gate. The pull-down mechanism is operatively connected to a pulley located on the other side of the plate which is driven by a belt which in turn is driven by a drive pulley located off the plate. Although spring loaded mechanical gearing could be used to drive the pull-down mechanism, the belt driven embodiment described above provides a particularly easy way to change the position of the pull-down mechanism pulley with respect to the drive pulley without having to provide for gear interface adjustments.
A further embodiment provides for the drive pulley to also drive rotating film carriage sprockets located above and below the film gate. During normal projection, a film loop is provided between the upper sprocket and the film gate thereby providing the slack necessary for each intermittent pull-down cycle. If film direction is reversed, the upper sprocket pulls the film upward through the film gate and another film loop is formed between the pull-down mechanism and the film gate. The belt driven upper sprocket provides a continuous pulling of the film through the film gate, thus eliminating film breakage often experienced when the upper sprocket acts only as an idler gear and the pull-down mechanism itself is used to push the film upwardly through the film gate.